Nitrous oxide (dinitrogen oxide, N2O) makes a substantial contribution to the greenhouse effect. The global warming potential (the extent to which a molecule contributes to the greenhouse effect compared to one molecule of CO2) of N2O is approx. 310. For a number of years, the policy of reducing emissions of greenhouse gases has been developed. The present invention can make a significant contribution to this policy. Various significant sources of N2O emissions have been identified: agriculture, industrial production of nylon precursors (adipic acid and caprolactam), the production of nitric acid and motor vehicles fitted with a three-way catalyst.
In principle, various catalytic and non-catalytic techniques can be employed in order to render nitrous oxide harmless. Various catalysts are known for the direct catalytic decomposition of N2O to N2 and O3 (cf. the literature summary provided by Kapteijn et al., Appl. Catal. B9 (1996), pp 25-64 and U.S. Pat. No. 5,171,553). However, this reaction is hampered to a considerable extent by the presence of oxygen and water, which are to be found in the off-gases from virtually all the N2sources listed above. Selective catalytic reduction is a promising alternative. Various catalysts for the conversion of N2O with the aid of olefins (CnH2n), alcohols or ammonia have been studied in the literature (recently: Mauzevin et al. Appl. Catal. B23 (1999) L79-L82 and Pophal et al. Appl. Catal. B16 (1998) pp. 177-186 and the literature cited therein ). Catalysts employed are often zeolites which have been substituted with a transition metal, such as iron, cobalt or copper.
For both technical and economic reasons, the addition of saturated hydrocarbons (CnH2n+2) would be preferable to the abovementioned reducing agents. Natural gas (CH4) and LPG (mixture of C3H8 and C4H10) are particularly attractive in this context. It is important that the formation and emission of carbon monoxoide (CO) and emission of unreacted hydrocarbons be minimized.
The present invention relates to a catalyst which enables N2O to be converted into nitrogen at a relatively low temperature and which allows very low emissions of CO and hydrocarbons to be achieved.